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Site Characteristics and Design Parameters
Figure 2 illustrates the locations of the three NAPL Areas. The depth to groundwater varied slightly in each area but was on average about five feet below ground surface (bgs).
NAPL Area 1 measured approximately 25,400 square feet and the remediation system included 106 ERH electrode and co-located MPE locations. Treatment depth in Area 1 extended from 2 feet to approximately 38 feet below grade (bg), producing an estimated treatment volume of 35,040 cubic yards.
In NAPL Area 2, a total of 22,390 square feet of subsurface was heated to a depth of 43 feet bg, cleaning a total estimated volume of 36,000 cubic yards. There were 101 electrode and co-located MPE locations in Area 2.
NAPL Area 3 measured approximately 18,200 square feet. The ERH remediation system included 90 electrode and co-located MPE locations and remediated soil and groundwater to a depth of 30 feet bg treating a total volume of approximately 20,200 cubic yards.
Heating of NAPL Area 1 started in late December 2003 and was completed in August 2004. Energy was first applied to NAPL Area 2 in January 2005 and remediation was complete by early August 2005. The ERH remediation in NAPL Area 3 began in early fall 2006 and was completed in January 2007. Results for Areas 1, 2 and 3 are presented below.
In addition to the ERH/MPE components, the remediation systems included hydraulic control wells to help depress the groundwater table and reduce the flow velocity within the treatment areas, a liquid waste management system (LWMS) to remove NAPL and to treat recovered groundwater, and a 1,000-scfm thermal oxidizer to treat vapors recovered from the subsurface.
In order to evaluate the performance of the ERH project, the ACOE set forth the following contaminant source area performance criteria:
- Minimize the time to implement the remedy while maximizing mass removal,
- Establish, maintain, and verify control of contaminant migration in groundwater, vapors, and air emissions,
- Establish and verify subsurface temperatures of 90ºC in the vadose zone and 100ºC in the saturated zone,
- Maintain target subsurface temperatures for set time intervals, and
- Provide a system for near real time data delivery, performance and compliance monitoring, and project communications.
Near real time project management was achieved by the use of daily, weekly, and monthly reports regarding daily sample and process monitoring data. Reports were presented in electronic format on a dedicated project website, enabling the Army, ACOE, Environmental Protection Agency (EPA), U.S. Army Environmental Center (AEC), and the TRS remediation team to analyze and monitor the progress of the remediation in near real time.
The project team managed the site using a Triad approach that combined flexible design and work plans, rapid data turnaround times, and the web based communication system. This project management approach allowed the team to incorporate nationally recognized research programs on the in situ biological aabiotic destruction of TCE into the projeThe Triad management approach also helped the project team successfully handle a series of challenges related to site conditions at the EGDY.
Because the site lithology is complex, final electrode depths were determined in the field based upon actual site conditions. To maintain an aggressive production schedule, TRS used air rotary drilling methods to install most of the electrodes, but utilized sonic drilling and continuous sampling at selected locations to allow further delineation of the contaminant plume. The result was that an unexpected area of DNAPL impact in Area 1 was discovered during electrode installation. The project team was able to rapidly expand the ERH and monitoring systems to include this DNAPL area without delays in schedule and with very little impact to the project budget.
During the final stages of the design process, two improved methods to treat and dispose of liquid effluents were evaluated. The Army accepted both alternatives and TRS implemented them without delays in schedule and at an overall savings to the project. During operations in NAPL Area 1 significant quantities of NAPL were recovered to the surface. Figure 3 shows a mass of NAPL collected at the bottom of the vapor/liquid separator before the separator reached temperature. Once the separator reached temperature, the NAPL continued to flow through the recovery equipment as a liquid and was then treated in the liquid waste management system.
Results of ERH Remediation
Figure 4 shows the subsurface temperatures in NAPL Area 2 in June 2005, several weeks before subsurface heating ended. Subsurface temperature readings were automatically uploaded daily to the onsite control computer.